1. Field of the Invention
The present invention relates generally to a circuit for regulating current in a light-emitting diode (LED).
2. Description of the Related Art
Light-emitting diodes (LEDs) are used in various display devices including LED video billboards. Display devices such as LED video billboards may include a large number of LEDs to produce high resolution images or videos. Brightness of the LEDs in such display devices fluctuate in response to current in the LEDs. Especially in large LED display devices, minor changes in their operating currents may result in flickering visible to human eyes. Therefore, the current in the LED must be regulated by a LED driver circuit to maintain the current constant in the LED.
LED driver circuits may be used to control one or more LEDs. The LED driver functions as a current source or a current sink that regulates current in an LED despite changes in voltage conditions or variations in other operating conditions. Typically, the LED driver circuits consist of digital components that communicate with other digital circuitry in a display device and analog components for controlling the current in the LEDs. The LED driver circuits may be designed to include multiple channels, each channel controlling an LED according to signals received from other digital circuitry in the display device.
FIG. 1 is a circuit diagram of a conventional LED driver implemented by a current mirror. The LED driver of FIG. 1 includes a current source 104, an input stage, a DC voltage source 110, an LED 108 and an output stage. The input stage of the LED driver in FIG. 1 includes MOSFET (metal-oxide-semiconductor field-effect transistor) MI1 and MOSFET MI2. MOSFET MI2 is connected between MOSFET MI1 and ground (GND). The output stage includes MOSFET MO1 and MOSFET MO2. MOSFET MO2 is connected between MOSFET MO1 and ground (GND). The current source 104 and the LED 108 are connected to MOSFET MI1 and MOSFET MO1, respectively. The DC voltage source 110 is connected to the gates of MOSFETs MI1 and MI2 to provide constant gate voltage to MOSFETs MI1 and MO1. The current source 104 provides a reference current Ii to the input stage. In response, the output stage produces output current Io by the well-known operation of the current mirror (comprised of MOSFETs MI1, MI2, MO1 and MO2).
In the LED driver of FIG. 1, MOSFETs are cascaded in the input stage and the output stage to alleviate or remove the short channel effect of MOSFETs MI2 and MO2. FIG. 2 is a graph illustrating the short channel effect of a non-cascaded MOSFET. As illustrated in FIG. 2, a drain-source voltage difference VDS of the MOSFET causes current IDS from the drain to the source of the MOSFET to change because of the short channel effect. That is, as the drain-source voltage difference VDS in MOSFET increases, current IDS in the MOSFET increases even in the saturation region. Since the operating conditions or resistance of the LED may cause drain-source voltage difference VDS to change, the current IDS may vary accordingly. When such MOSFET is used to operate an LED, the changes in the current IDS result in changes in the brightness or flickers in the LED. Hence, many LED drivers adopt a cascaded MOSFET structure as illustrated in FIG. 1 to provide consistent output current Io to the LED 108 despite variations in the drain-source voltage difference VDS.
However, cascaded MOSFETs in the LED driver take up a large space in an IC (integrated circuit) chip, especially when attempting to implement a LED driver with a low operating voltage. The increased space occupied by the MOSFETs poses challenges and issues in miniaturizing the IC chip or increasing the number of channels in the IC chip.